The SMR process is mainly based on the reforming reaction of light hydrocarbons such as methane that yields to a mixture of hydrogen (H2) and carbon monoxide (CO) in the presence of water vapor. The main reaction is endothermic and slow and requires additional heat input, as well as a catalyst to occur. Usually, SMR reactor performances are limited by the heat transfer and not by the kinetic of the reactions.
In industrial practice, the SMR reactor usually comprises tubes placed in a furnace, said tubes being filled with catalyst, most often in the form of pellet, and fed with the process gas mixture (usually methane and steam).
Several well-proven configurations are available for furnace design as illustrated by the FIG. 1 which presents top fired (also known as down fired), bottom fired (also known as up fired), side fired, and terrace wall.
The top-fired technology is one of the most referenced designs and it is proposed by several technology providers. Top-fired furnaces are typically made of a refractory lined firebox containing several rows of catalyst containing tubes. The necessary heat for the endothermic reaction to occur is provided by roof burners placed in rows between the tubes, and also by rows of additional roof burners at the furnace side, along the walls of the furnace. The combustion products out of the burners are usually blown vertically downwards, so that the tube rows face the flames in their upper part. A flue gases exhaust collector is usually provided at the furnace floor level.
The bottom fired technology is less common in modern plants. According to the bottom fired technology, the burners are arranged in row on the floor of the firing area between the tube rows and fire vertically upwards.
The main objective of the furnace design (also called firebox design) is to maximize the heat transferred from the burners to the tubes—from the burner flames and also from the walls and the hot flue gas while respecting a tube maximal operating temperature constraint. The tube maximal operating temperature or MOT (also known as maximal operating constraint or MOT) is a function of several factors, and particularly of the tube mechanical load (mainly feed gas pressure), of the mechanical properties of the alloys used for the tubes and of the desired lifetime of the tubes exposed to creep and thermal aging.
Any intensification of the heat transferred to the tubes has a direct positive impact, either by increasing the productivity or by improving the compactness of the firebox which is valuable in terms of capital expenditures. However, intensification of the heat transferred usually implies higher tube skin temperature levels that reduce tube lifetime or require use of more resistant alloys, which are much more expensive.
Lack of homogeneity in the heat duty distribution in the furnace will lead some of the tubes to be hotter than other ones; temperature profiles of tubes are therefore critical elements for the design of the furnace and during operation. Tube temperature profiles provide decisive information when looking for good compromise between performance and durability, a good compromise being actually essential.
During operations, the performances of the furnace are therefore limited by the temperature of the hottest tube; it should not be hotter than the MOT. In the meantime, the process performance i.e. the productivity depends on the average tubes heat flux and temperatures. Therefore, the smaller is the difference between the hottest tube temperature and the average tube temperature; the better is the furnace performance.
Seeking for simplicity, most of the explanations that follow are made with regards to a top fired furnace. However, it is to be noted that figures and explanations with regards to a bottom fired furnace would be comparable.
In such a top fired furnace, as shown on FIG. 2, the catalyst containing tubes are arranged in rows within the furnace. The feed is supplied through the top part of the tubes; the synthesis gas produced—containing hydrogen and carbon monoxide as major components, and several minor components and traces—is withdrawn at the bottom part of the tubes. Burners are arranged in rows between the tubes rows and between tubes and walls. Resulting flue gases are extracted through exhaust tunnels.
FIG. 3 presents a top view of the same top-fired furnace showing 8 rows of 48 tubes each row being organized in 3 sections (bays) of 16 tubes each—and 9 rows of 15 burners arranged as well in 3 sections (bays) containing 5 burners each, and parallel to the tubes rows. The rows of burners are ended by a wall (wall along Y axis also identified as “end walls”). For all rows of tubes, the end tubes facing the end wall are identified as “wall end tubes”.
For each row of tubes or burners, the high number of tubes and/or burners in each row induces geometrical constraints in the furnace that makes it necessary to add support beams to ensure safety of the furnace; said supports therefore divide the rows of tubes and the rows of burners as well in several sections (also known as bays) periodically repeated. Each section end either by an end wall or by a symmetry plane—plane that is in middle of the space left between two adjacent sections to allow the installation of the supports. The end tubes closest to the symmetry planes are identified as “symmetry end tubes” or “symmetry tubes”.
The expressions “outer section tubes” or “outer tubes” refer to “wall end tubes” and “symmetry end tubes” without making a distinction between them.
All tubes that are not “wall end tubes” or “symmetry tubes” are identified as “inner section tubes” or “inner tubes”.
The presence of the end walls close to the “wall end tubes” and the division of the tubes rows in sections—therefore creating a different space between two particular adjacent tubes—lead to inhomogeneous repartition of the available heat between the “wall end tubes”, the “symmetry end tubes” and the “inner tubes”.
In all the description the expression “row of burners” is to be understood as “row of burners parallel to the tube rows”, this direction of the rows being also identified as X axis.
In the furnaces to which the invention applies, i.e. with burners placed in rows parallels to the tube rows, for each burner the direction of the flame jet created by the burner is affected by:                the interaction with nearby co flowing jets, and        the presence of wall (if any) that could also lead to an inhomogeneous repartition of the heat among the tubes belonging to the same row.        
Inhomogeneity of heat distribution among tubes within a row that comes from the flame jets interaction within a row of burners parallel to the tube rows (along X-axis) has previously been considered; a solution has been found and disclosed in U.S. Pub. 2018/0372310 that solves the problem of over (or under) heating of tubes that comes from the repartition of the burners within the row of burners adjacent to the tube row. This type of in homogeneity of heat distribution among tubes is therefore not considered in this invention.
However, there remains a problem of inhomogeneity of heat distribution that concerns mainly outer section tubes and is not solved by the above cited patent application.